This invention relates to fried tofu products and processes for making same.
"Tofu" is one of the most popular traditional soybean foods in Eastern Asia and is an important protein source for people living in this region. It is a watery, white, jelly-like fragile product having a moisture content as high as 88 weight %. In most cases, tofu is prepared by complicated manual processes. In a typical process, soybeans, as the raw material, are soaked in water for prolonged periods, (e.g., about 8 hours during the summer months and about 22-24 hours during the winter months) and the soaked soybeans are ground to form a slurry thereof. This slurry is heated to extract the soy protein (i.e., the soy protein becomes dissolved in the water phase) and then filtered through a coarse cloth to remove insoluble soybean refuge, consisting mainly of pulverized cellular substances commonly like "okara". The filtrate or soy milk is coagulated to form a curd by adding a coagulating agent, such as a non-toxic water soluble alkaline earth metal salt (e.g., calcium sulfate, calcium chloride or magnesium chloride) and the whey is separated from the tofu curd by draining. In recently developed processes, certain organic gamma or delta lactones of aldonic and uronic acids, such as D-glucono-.delta.-lactone, are used as the coagulating agent and the necessity for whey draining is eliminated. Notwithstanding this improvement, prior processes used for making tofu still require many laborious, time-consuming steps and generally are not adaptable for automated mass production.
Tofu is used as a foodstuff in several different forms. In Eastern Asia, it is commonly consumed as a fried product. Today, about one third of the tofu consumed in Japan is in this form.
"Aburage" (often referred in shortened form as "age") is one of the most common forms of fried tofu. It is generally square shaped and is prepared by slicing tofu curd into relatively thin slices, pressing the slices to adjust the moisture content therein to about 80-82% and then deep frying in two successive stages. Aburage is consumed chiefly in a cooked state or in a toasted state with appropriate condiments and/or seasonings.
Occasionally, aburage is also consumed as an edible casing or wrapping for other foodstuffs. This aburage casing is prepared by diagonally cutting a square piece of the aburage to form two triangular pieces. Each triangular piece is then slit open along the cut edge to form a bag. "Sushi" is prepared by stuffing the aburage bags with various foodstuffs. For example, "inarizushi", a particular kind of "sushi", is prepared by stuffing the aburage bags with a vinegared rice. Daily dishes can be prepared by stuffing bags formed from aburage with various comminuted vegetables and fastening the stuffed bags with an edible string or cord, such as "kanpyo" (made from the fruits from certain convolvulacean and lagenarian plants). Aburage can be cut into two rectangular pieces, each of which are slit open along the cut edge to form a bag which is used in a similar manner.
Another type of fried tofu is "ganmodoki", also called "hirosu", which is prepared by crushing or grinding drained tofu curd, mixing a curd binder, such as dioscorean yams (e.g., D. janponica thunb. or D. betatas decne.), and minced vegetables with the ground tofu, forming this mixture into a rounded disc shape, and then deep frying the shaped mixture in two successive stages. The product is consumed in a manner similar to aburage.
Because of the laborious and time-consuming manual steps typically required by these methods for preparing fried tofu products, economical mass production of such products has not been possible in the past. Tofu formed by the lactone coagulants mentioned above generally are not acceptable for preparing a fried tofu product because of the difficulty of draining whey to reduce the moisture content to the desired level of 80-82%. Also, the tofu is acidifed by these coagulants and tends to reduce the flexibility of the fried product. Consequently, preparation of fried tofu products generally are restricted to the use of a tofu curd formed with an alkaline earth metal salt. The presence of these salts in the fried product has several disadvantages.
First, it is often desirable to freeze the fried fu products for prolonged storage. This is particularly true for ganmodoki which retains a considerable amount of moisture even after frying (because of its thickness as compared to the thin aburage) and therefore, tends to putrefy within a relatively short time unless frozen. While being stored in a frozen state, the alkaline earth metal salts present in the fried products tend to accelerate the denaturation of soy protein. This denaturation causes the thawed product to have a porous texture (i.e., honeycomb-like) and unsatisfactory mouthfeel or chewiness.
Second, unless extreme care is exercised during frying, these salts tend to leach out onto the frying oil and cause a deterioration thereof.
Third, it is desirable to incorporate egg ingredients (egg yolk, egg white, or whole egg) into the fried products to improve their flavor and/or food value. It is difficult to do this with prior processes, particularly when aburage is being prepared. If the egg ingredient is added to the heated soybean slurry prior to filtering, the egg protein may be filtered out with the soybean refuge because it tends to coagulate at temperatures below which the slurry must be heated to extract soy protein. If the egg ingredients are added to the soy milk before curd formation, some of the egg protein may not be coagulated by these salts and, consequently, is drained away along with the whey. Therefore, the above-discussed prior processes generally are not acceptable for preparing fried tofu products (particularly aburage) also containing egg ingredients.